1. Field of Invention
The present invention relates to a scanning electron microscope and more particularly to calibration of a scanning electron microscope to correct for the effects of local charging.
2. Description of Related Art
In the operation of a Scanning Electron Microscope (SEM), the incident electron beam can cause local charging (i.e., a buildup of charge) on the sample. Local charging, in turn, can influence the incident beam and significantly distort measurements taken by the SEM. The operation of an SEM requires the focussing of an incident electron beam onto a substrate. Backscattered electrons, as well as secondary electrons generated by the incident beam, are then collected for measurement purposes. By scanning the incident electron beam across a feature of interest and synchronously detecting the backscattered and secondary electrons, a measurement of the feature""s size, known as a critical dimension (CD), can be obtained. A CD can be any characteristic feature to be measured. For example, a pitch CD denotes the repeat distance of a periodic structure such as a series of parallel lines.
The accuracy of a CD measurement is dependent on how accurately the SEM scanning operation can be calibrated. As illustrated in FIG. 4, an electron beam 61 is focussed onto a substrate such as a wafer 63. The actual distance covered on the substrate by the incident beam, which is known as the scan length, can be affected by the local electrostatic potential at the substrate surface. Under nominal conditions, the higher the local electrostatic potential, the shorter the scan length. Beam-induced local charging occurs when the number of electrons leaving the substrate (from backscattering and secondary electron generation) is different from the number entering it (from the incident beam). This affects the local electrostatic potential at the substrate surface, and thus the scan length. In FIG. 4 the absence of local charging leads to a scan length 65. When local charging is present on an area 67 of wafer 63, a resulting electric field 69 leads to a scan length 71 due to the change in the electrostatic potential.
This variation in the scan length due to local charging will lead to CD measurement inaccuracies since the scaling of distances depends on the scan length. That is, as the local charging varies, the ratio of a fixed CD to the scan length will correspondingly vary. This ratio provides a scaling that determines the CD measurement.
Thus, two wafers with identical features may lead to different scan lengths because of differences in local charging. Similarly, if the magnitude of local charging varies from substrate to substrate, the scan lengths will also vary. Compensating for these local-charging induced changes in scan length is necessary to maintain CD measurement accuracy across a wide variety of substrates and operating conditions.
Typically, the calibration of an SEM has been limited to an adjustment so that a measured CD matches a given value for the CD (e.g., U.S. Pat. No. 4,818,873). Correcting for errors associated with local charging has then generally been limited to parametric calibrations based on operational factors such as the composition of the substrate and the voltage levels used. That is, a different SEM calibration is required for each distinct operational setting and each type of substrate to account for the associated errors. This cumbersome approach requires considerable attention from a user in order to obtain a calibrated measurement. In addition, calibration to remove errors due to local charging is especially difficult to accomplish in operational settings exhibiting a high degree of sensitivity.
Calibration errors associated with local charging may also be difficult to detect since they are often not accompanied by any measurable change in image focus. However, the magnitude of these calibration errors in linear measurements may be on the order of 2-4% under nominal operating conditions, an error that is generally considered to be unacceptably large.
Accordingly, it is an object of this invention to provide a method and associated system for calibration of an SEM to correct for errors due to local charging.
It is a further object of this invention to provide a calibration of an SEM in a way that automatically accounts for variations in local charging.
It is a further object of this invention to provide a calibration for an SEM in a way that is simple and relatively transparent to the user.
It is still a further object of this invention to provide a calibration for an SEM by measuring the local landing energies to characterize the local charging for a calibration wafer and a measurement wafer.
It is another object of this invention to provide a calibration for an SEM by using measurements of local landing energies to extend the applicability of an SEM calibration.
The above and related objects of the present invention are realized by a system and method for calibrating a scanning electron microscope with respect to a calibration wafer, measuring a local landing energy of the calibration wafer with the microscope, measuring a critical dimension of the measurement wafer with the microscope, measuring a local landing energy of the measurement wafer with the microscope, and calculating the calibrated critical dimension of the measurement wafer.
Calibrating the microscope may be carried out by measuring a critical dimension of the calibration wafer, comparing the measured critical dimension of the calibration wafer to a reference critical dimension, and adjusting a scan calibration value of the microscope.
Measuring the local landing energy of a wafer may include determining the electrostatic potential at the substrate surface. In a preferred embodiment, measuring the local landing energy of a wafer includes measuring the energy of backscattered and/or generated secondary electrons.
Preferably, calculating the calibrated critical dimension of the measurement wafer includes scaling the measured critical dimension of the measurement wafer by a scaling factor, where the scaling factor is determined from the measured local landing energy of the calibration wafer and the measured local landing energy of the measurement wafer. The scaling factor may be used to determine a calibrated scan length for the microscope by scaling a reference scan length by the scaling factor.
The scaling factor may be determined by an evaluation of a scaling function for a ratio of the measured local landing energy of the measurement wafer with respect to the measured local landing energy of the calibration wafer. The scaling function is approximately one when the ratio is approximately one. The scaling function should have a simple structure such as a linear or a higher-order polynomial.
The present invention possesses a number of distinct advantages over known calibration systems. A correction for the effect of local charging on an SEM can be accomplished in a way that is simple and relatively transparent to the user. The LLE can be measured by in-line electron optics with relatively limited hardware additions to the basic SEM. The scaling of the measured CD of the measurement wafer is a relatively simple software operation. A scaling function with simple structure, such as a linear polynomial, can be tailored for this purpose.
This calibration tool provides an important enhancement to SEM technology since the effects of local charging on a CD measurement of a wafer may not be readily apparent to the user. Correcting for these errors without incorporating measurements of local charging as in the present invention may unavoidably require a difficult parametric calibration process.
These and other objects and advantages of the invention will become more apparent and more readily appreciated from the following detailed description of the presently preferred exemplary embodiment of the invention taken in conjunction with the accompanying drawings.